> Maybe make a 1/4 wave ground plane instead of a dipole? The ground
would connected to a set of 4 or more
> ground radials down tilted 45 degrees and a single straight up and
down stick. http://ccarc.org/_misc/so-239_ant.html
<http://ccarc.org/_misc/so-239_ant.html>
Yes. This is a dead simple antenna that works pretty well. It's designed
to be fed unbalanced, just like the diagram. You can adjust the
impedance match (and the power out) by bending the four lower diagonals
closer to or farther away from the coax, although 45 degrees is usually
about right. You can use any panel mount connector (BNC, N, etc.) and,
if the connector is made of the correct material, you can just solder
the lower wires to the base of the connectors, rather that use screws as
the diagram shows. Also, the 14-guage conductors from the inside of a
piece of Romex work well as the radiating elements.
Kevin
On 9/29/20 1:18 AM, Tadd Torborg via TriEmbed wrote:
Maybe make a 1/4 wave ground plane instead of a dipole? The ground
would connected to a set of 4 or more ground radials down tilted 45
degrees and a single straight up and down stick.
http://ccarc.org/_misc/so-239_ant.html
<http://ccarc.org/_misc/so-239_ant.html>
I was thinking about what you said about stepping the frequency as the
car moved. Would successive cars entering the chain have different
frequency selections?
I wonder if you couldn’t choose a different frequency for the actors
49mhz, 900mhz, 2.4ghz or something, and have a central site that had a
separate receiver for each microphone, and then mix the audios
together into a single FM transmitter. I suspect the one-way wireless
mike to receiver has been done and you may even find somebody’s
maker-space article on the subject. Make 5 of those for your five
actors, then a single FM transmitter with the decent antenna you are
discussing would talk to the cars throughout the entire drive-through.
Tadd - KA2DEW
On Sep 28, 2020, at 10:31 PM, Pete Soper via TriEmbed
<[email protected] <mailto:[email protected]>> wrote:
I modeled 1/2" diameter elements with EZNEC and the length of each
1/4wl element comes out to 29.1 inches when cut for 98MHz with the
antenna 10 feet up.
The graph of SWR vs frequency from 88 to 108MHz is below.
<http://triembed.org/images/FM-DIPOLE.png>
The closer to resonance the lower the SWR and greater proportion of
RF out vs turning the power into heat. So if you end up needing to
transmit at, say, 89MHz you'd simply make the elements 98/89 of the
above dimension and it should keep the SWR as low as it can get. If
you were transmitting at 107MHz you'd multiply by 98/107.
But the other problem with going unbalanced into a dipole is that it
doesn't necessarily radiate like a dipole (i.e. two lobes
perpendicular to the elements). As Dan mentioned, the feedline ends
up radiating and it tends to be at wonky angles relative to the axis
of the antenna elements. The coax coil (or purpose-made balun below)
solves this by isolating the feedline from the antenna. It's a pain
to have the antenna only be effective for a piece of the azimuth
range you need. From the description you probably want an omni
pattern. The gain off the ends of a horizontal dipole is terrible and
the gain falls off pretty severely more than around 40 degrees right
or left of the broadside direction. So in addition to a balun you
might consider making the dipole vertical. A vertical dipole is
omnidirectional outward with the nulls up and down. But the balun is
key to getting a predictable pattern.
You can get baluns from Digikey, by the way. The MABA-011040
<https://www.digikey.com/product-detail/en/macom-technology-solutions/MABA-011040/1465-1702-1-ND/5131112>
is rated for 1-300MHz and doesn't look like it would be hard to
solder. This could go at the antenna and then you'd use a simple run
of coax to it.
-Pete AD4L
<opiadpmmadeneekh.png>
On 9/28/20 12:29 PM, Brian via TriEmbed wrote:
Hi Folks,
I know there are several radio-smart people on this list, so I hope
the rest will forgive the noise as I pose a couple questions here.
The questions first; I'll provide background afterward.
I have settled on a digital FM transmitter chip (Silicon Labs'
SI4721), and a simple dipole antenna. Two questions arise from this
decision:
1. The transmitter has a single-ended RF output, but will be fully
isolated from earth ground in operation (running on batteries or an
isolated AC/DC supply). A dipole is a balanced load, but since
"ground" of the radio circuit is isolated, can I just treat it as
"balanced" and connect the circuit ground to the other half of the
dipole? Or do I really need to use a balun for a proper balanced
output? Coverage area actually needs to be very small (< 100'), so
I'm not majorly concerned with impedance mismatch losses, etc.
2. I'll be using 1/2" copper pipe as the elements, held inside a
larger PVC enclosure. What's the best way to bond wires to the
pipe? Should I just solder them on? Tap a hole and use a screw to
clamp them? Some kind of shark-bite approach? Does it even matter
at all?
Here's the background:
My church does a Christmas program called the Drive-Thru Christmas,
which is made up of five live-actor scenes distributed around our
parking lot. Guests are typically given a narration on CD which
they play inside their vehicles as they move from scene to scene.
In order to improve our social isolation this year, I'm doing some
R&D on the "talking sign" idea, using five separate short-range FM
transmitters to broadcast the scene's narration to the guest's FM
radio in their car. Each transmitter would broadcast on a different
frequency, and some system would step each transmitter through the
list of frequencies in time with the car's movement through the
scene, so we can maintain our 5-car pipeline but not require the
guest to re-tune their radios. We tried using an internet streaming
option last year (for folks with smartphones linked to their car
stereos) but that, I hear, was an abject failure with many people
unable to access the stream.
Well anyway, thanks in advance for any advice!
Cheers,
-Brian
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